1. Field of the Invention
The present invention relats to a recursive noise reducer for reducing a noise of a digital video signal and to a noise reducing method thereof. The present invention also relates to a noise reducer, a noise reducing method, a picture signal processing apparatus, and a motion detecting method suitable for recording/reproducing a picture to/from a record medium such as an optical disc or a magnetic tape and for transmitting a picture from a transmitting side to a receiving side through a transmission path in a television conference system, a television telephone system, a broadcasting unit, and so forth.
2. Description of the Related Art
A recursive noise reducer using a frame memory has been well known in the field of digital picture signal process. The recursive noise reducer obtains the differences between an input video signal and a video signal of one frame prior received from the frame memory, extracts portions having small levels from the differences as noise components, and subtracts the extracted noise components from the input video signal. Thus, the recursive noise reducer reduces the noise of the input video signal and writes the resultant video signal to the frame memory. When a field memory is used instead of the frame memory, the capacitance of the memory can be decreased.
FIG. 1 is a block diagram showing an example of the structure of a conventional noise reducer. An input video signal that has been converted into a digital signal (referred to as input video signal Vin) is received from an input terminal 201. The input video signal Vin is supplied to subtracting devices 202 and 204. An output signal of the subtracting device 202 is obtained from an output terminal 207. In addition, the output signal of the subtracting device 202 is written to a frame memory 205. A memory controller 206 is disposed corresponding to the frame memory 205. The memory controller 206 controls a writing operation and a reading operation of the frame memory 205. The read data of the frame memory 205 is delayed by one frame against the write data. An output signal of the frame memory 205 is denoted by Vout. One frame delay is denoted by Fxe2x88x921. Thus, the output signal Voutxc2x7Fxe2x88x921 of the frame memory 205 is supplied to the subtracting device 204. Since the input picture signal Vin is supplied to the subtracting device 204, the subtracting device 204 generates a frame difference.
The output signal of the subtracting device 204 is supplied to the subtracting device 202 through a non-linear circuit 203. The non-linear circuit 203 multiplies the output signal of the subtracting device 204 by feed-back coefficient K corresponding to the level of the input picture signal. The non-linear circuit 203 is composed of a ROM. As input/output characteristics of the non-linear circuit 203, when the level of the input signal (frame difference) is small, feed-back coefficient K=1 is set and the input signal is output as a noise component. When the level of the input signal is intermediate, the level of the output signal is limited to a predetermined value. When the level of the input signal is large, the output signal is decreased. When the level of the input signal is much large, assuming that a frame difference takes place due to a motion, the level of the output signal is 0.
Thus, using the characteristics of which the inter-frame correlation and the amplitude of a noise component are small, the non-linear circuit 203 extracts the noise component. The subtracting device 202 subtracts the extracted noise component from the input video signal. Thus, the noise can be reduced.
The output video signal Vout can be expressed by the following formula.
Vout=Vinxe2x88x92Kxc2x7(Vinxe2x88x92Voutxc2x7Fxe2x88x921)=Vinxc2x7(1xe2x88x92K)/(1xe2x88x92Kxc2x7Fxe2x88x921)xe2x80x83xe2x80x83(1)
In the above-described noise reducer, when there is a moving portion on a picture, the inter-frame correlation is lost. Thus, since it becomes difficult to distinguish a noise from an input signal, a moving blur takes place. To solve such a problem, a noise reducer (for example, Japanese Patent Laid-Open Publication No. 7-15630) has been proposed. In the noise reducer, an inter-frame or inter-field moving vector in a particular direction of the entire picture is detected. The delay amount of the memory is controlled corresponding to the amount of movement.
In addition, a method for improving the prediction accuracy of the motion compensation of an encoder (for example, Japanese Patent Laid-Open Publication No. 6-296278) has been proposed. In the method, a pre-processing apparatus of a motion compensation prediction encoding apparatus performs a motion compensation predicting process for an input picture that has a large moving portion and contains many random noise components.
In the noise reducer (Japanese Patent Laid-Open Publication No. 7-15630), which performs the motion compensating process, although a moving blur due to a parallel movement of a whole picture can be alleviated, a moving blue due to a plurality of moving portions on a picture cannot be alleviated.
On the other hand, in the method (Japanese Patent Laid-Open Publication No. 6-296276), since the motion compensating process is performed block by block, a block distortion of which the boundary of a block is conspicuous on a decoded picture takes place. In addition, to obtain an inter-frame moving vector, a frame memory is required. Thus, the cost of the apparatus increases.
Therefore, an object of the present invention is to provide a noise reducer, a noise reducing method, and a picture signal processing apparatus for suppressing a moving blur without a block distortion even if a plurality of moving portions take place on a picture.
Another object of the present invention is to provide a noise reducer, a noise reducing method, and a picture processing apparatus for dividing an interfield difference or an inter-frame difference into a plurality of frequency components so as to properly reduce noise.
A further object of the present invention is to provide a motion detecting method for detecting an inter-field motion or an inter-frame motion for each of a plurality of moving portions.
A first aspect of the present invention is a noise reducer, comprising a moving vector detecting means for dividing a picture signal into a plurality of blocks and detecting moving vectors of the blocks between adjacent fields or adjacent frames, a motion compensating means for extracting a moving portion of a picture of one field prior or one frame prior corresponding to the moving vectors and compensating the motion of the extracted moving portion corresponding to the moving vectors, a difference signal obtaining means for obtaining a difference signal between the picture signal and a signal that has been motion-compensated of one field prior or one frame prior, a non-linear process means for performing a non-linear process for the difference signal, and a combining means for combining the picture signal and the signal that has been non-linear-processed.
A second aspect of the present invention is a noise reducer, comprising a moving vector detecting means for dividing a picture signal into a plurality of blocks and detecting moving vectors of the blocks between adjacent fields or adjacent frames, a motion compensating means for compensating the motion of a picture of one frame prior or one field prior with the moving vectors, a difference signal obtaining means for obtaining a difference signal between the picture signal and a signal that has been motion-compensated of one field prior or one frame prior, a dividing means for dividing the difference signal into a plurality of frequency component difference signals, a non-linear process means for performing a non-linear process for the individual frequency component difference signals, and a combining means for combining the picture signal and the frequency component signals that have been non-linear-processed.
A third aspect of the present invention is a picture signal processing apparatus for compression-encoding a picture signal, comprising a noise reducer for reducing a noise of a picture signal that has not been compression-encoded, wherein the noise reducer comprises a moving vector detecting means for dividing a picture signal into a plurality of blocks and detecting moving vectors of the blocks between adjacent fields or adjacent frames, a motion compensating means for extracting a moving portion of a picture of one field prior or one frame prior corresponding to the moving vectors and compensating the motion of the extracted moving portion corresponding to the moving vectors, a difference signal obtaining means for obtaining a difference signal between the picture signal and a signal that has been motion-compensated of one field prior or one frame prior, a non-linear process means for performing a non-linear process for the difference signal, and a combining means for combining the picture signal and the signal that has been non-linear-processed.
A fourth aspect of the present invention is a picture signal processing apparatus for compression-encoding a picture signal, comprising a noise reducer for reducing a noise of a picture signal that has not been compression-encoded, wherein the noise reducer comprises a moving vector detecting means for dividing a picture signal into a plurality of blocks and detecting moving vectors of the blocks between adjacent fields or adjacent frames, a motion compensating means for compensating the motion of a picture of one frame prior or one field prior with the moving vectors, a difference signal obtaining means for obtaining a difference signal between the picture signal and a signal that has been motion-compensated of one field prior or one frame prior, a dividing means for dividing the difference signal into a plurality of frequency component difference signals, a non-linear process means for performing a non-linear process for the individual frequency component difference signals, and a combining means for combining the picture signal and the frequency component signals that have been non-linear-processed.
A fifth aspect of the present invention is a motion detecting method for dividing a picture signal into a plurality of blocks and detecting a motion between adjacent fields or adjacent frames of the picture signal, the method comprising the steps of detecting moving vectors of the individual blocks between adjacent fields and adjacent frames, obtaining the histogram of the moving vectors for one field or one frame, arranging the moving vectors in the order of higher frequencies of the histogram, and obtaining at least one moving vector with the highest frequency (frequencies), and detecting a block having a moving vector with the highest frequency as a moving portion.
According to the present invention (first aspect and so forth), a recursive noise reducer preforms a motion compensating process. Thus, a plurality of moving portions can be detected on a picture. By detecting moving vectors of the moving portions, a motion compensating process can be performed for each moving portion. Thus, the moving blur of the apparatus according to the present invention is smaller than the moving blur of an apparatus that equally performs a motion compensating process for a whole picture. According to the present invention (third aspect), since such a noise reducer is disposed upstream of a circuit that preforms a compression-encoding process, the efficiency of the compression-encoding process can be improved.
According to the present invention (second and so forth), an inter-field difference or an inter-frame difference is divided into a plurality of frequency components. A non-linear process is performed for each of the frequency components. Thus, in consideration of the conspicuousness of noise components corresponding to the frequency components, the noise components can be effectively reduced. According to the present invention (fifth aspect), the motions of a plurality of moving portions on a picture can be detected corresponding to moving vectors detected for individual blocks.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings.